It is very difficult to grow silicon carbide (SiC), group III-nitrides, and alloys thereof, with low impurity levels, high crystalline quality, and commercially-viable growth rates. The discussion below, for exemplary purposes only, focuses on the growth of SiC crystals, but this does not limit the scope of the invention.
SiC single crystals have unique electronic and physical properties, making them suitable for use in different types of semiconductor devices. SiC devices can operate at significantly higher temperatures than devices built using more conventional electronic materials such as silicon (Si) or gallium arsenide (GaAs). SiC has a very high electric breakdown field, making it suitable for high power communications devices operating in the microwave frequency spectrum. Furthermore, the thermal conductivity of SiC is significantly higher than that of Si and GaAs, allowing for a more efficient removal of thermal energy generated during the operation of semiconductor devices. This is particularly advantageous for high-power and high-frequency semiconductor devices.
To help improve the performance of SiC semiconductor devices in general, and microwave devices in particular, it is desirable to have low levels of background impurities in the SiC crystal. This is especially true for microwave devices, that use high-resistivity, semi-insulating substrates (with resistivity greater than about 1E05 Ω-cm) to help avoid problems associated with electrically-conductive substrates.
An approach to growing crystals is to replace the powder source in the PVT (Physical Vapor Transport) process with high-purity gases. High concentrations of precursor gases (such as silane and propane) are used instead of the powder source to produce Si- and C-carrying vapor species. For example, a mixture of silane, a hydrocarbon gas such as propane, and a carrier gas such as helium is pumped into the base of a cylindrical reactor that is heated externally, and that includes a wafer of SiC acting as a seed for crystal growth on the top flat surface of the cylindrical reactor. Silane decomposes to Six and H2, and because a high concentration of silane is used, Six clusters are formed. The Six clusters react with propane to form thermodynamically-stable Six-Cy clusters, and more H2. The gas stream containing the clusters enters a higher temperature region, where the clusters sublime to form vapor species containing Si and C (Si, SiC2, Si2C, and SiC). These vapor species are transported towards the growing surface by the bulk motion of the gas and decompose to form crystalline SiC on the lower-temperature seed surface. This method has been referred to as High Temperature Chemical Vapor Deposition (HTCVD) or Gas Fed Sublimation (GFS), and has been shown to produce high-purity materials at high growth rates. Compositional purity of materials grown by this technique is very high, orders of magnitude higher than that obtained in standard PVT-grown materials (carrier concentration of 1015 versus 1017 cm−3). The high purity of HTCVD materials may be related to the use of gas precursors instead of powder source material, that can be obtained at much higher purity levels than the solid source. Impurities such as boron and nitrogen, however, have been detected in the grown materials. The source of these impurities is believed to be the reactor walls and components.
At the elevated temperatures used in the GFS process for growth of SiC, graphite is believed to be the only material that is thermally, chemically, and economically suitable for construction of the reactor components, such as the containment walls, seed holder, etc. Although high purity graphite is generally used for the construction of reactors, the high processing temperature (2000–2500° C.) results in release of residual impurities from the graphite components. These impurities effuse into the gas stream and contaminate the growing SiC crystal. Impurities such as boron and nitrogen have been detected in the grown materials. The source of these impurities is believed to be the reactor walls and components. One approach to reduce the release of impurities from graphite components is to coat them with a high purity coating, such as SiC.
During GFS, the cracking and reactions of Si- and C-carrying precursor gases produce hydrogen. At high temperatures, hydrogen is highly reactive and reacts with silicon carbide, with the etching rate being significantly faster for polycrystals than single-crystals. Hydrogen has a beneficial effect at the growth surface, where it etches away polycrystals that may be formed on that surface, while leaving the single crystal portions less affected. In cases where the precursors do not produce any hydrogen or the amount produced is not enough, controlled amounts of hydrogen may be added to the gas mixture at the inlet to the reactor to control the etching action of the gas mixture. A deleterious side-effect of the presence of hydrogen in the reactor is its reaction with the silicon-carbide coating of the graphite reactor components, thus exposing the underlying graphite. The presence of nitrogen and boron in the GFS-grown SiC is related to the release of these impurities from the hot, exposed graphite.
In the GFS process, the reactive gas mixture is pumped into a reactor that is heated by an external source, typically radio frequency induction. The gas mixture is heated through contact with the hot walls of the reactor and subsequent diffusion of heat through the gas mixture. In general, the concentration of Si- and C-carrying precursors relative to the carrier gas is relatively low, and heat and mass transfer in the reactor is controlled by the thermophysical properties of the carrier gas. The thermophysical properties of the carrier gases which can be used (e.g. Helium, Argon etc) are such that heat diffuses at nearly the same rate as which hydrogen diffuses through the gas mixture. As the rate of gas-phase reactions increases rapidly with temperature (with a corresponding increase in the rate of release of hydrogen), by the time heat has diffused to the central portion of the gas mixture and heated it to the desired temperature, the formed hydrogen has diffused to the reactor wall, and started to etch away the silicon carbide coating of the reactor wall.
The above problems associated with release of impurities and diffusion of the impurities into the gas core are also present when the impurities are released from the reactor walls without the action of a component of the gas mixture on the reactor wall. Examples of such incidences would be the release of impurities from the reactor wall or its coating because of the high operating temperatures, or diffusion of impurities from other components in the reactor into the reactor wall and subsequent effusion into the gas mixture. In such circumstances, the diffusivity of typical impurities (for example nitrogen) is nearly the same as diffusivity of heat in the gas mixture.